A wide variety of iron-sulfur clusters and mixed-metal species are active sites in metalloenzymes. Our research focuses on the use of modern techniques of quantum chemistry and electrostatic modeling to carry out studies of the electronic structure of these centers, to more closely link the spectroscopy and energetics of these systems to their structure and function. A single important enzyme will be emphasized during the next project period. Nitrogenase is an enzyme complex of two proteins, an Fe protein and a MoFe protein, which in concert catalyze the multi-electron reduction of molecular nitrogen to two ammonia molecules. This bacterial enzyme is critical to the biological nitrogen cycle, allowing synthesis of ammonia under mild physiological conditions, which is an essential building block for the synthesis of proteins and nucleic acids within the growth cycle of many plants. An understanding of nitrogenase is important both for improvements in nitrogen utilization by plants and for the potential of developing improved synthetic nitrogen fixation processes. The catalytic function and regulation of nitrogenase is essential to the global biological nitrogen cycle, influencing the fertility of soils, and the ecological balance in wetlands, seas, and the oceans. Understanding the relationship of nitrogenase to other iron-sulfur centers will provide insights into the physical and biological principles governing these widespread redox and catalytic agents. We will focus on electronic structures, reaction pathway structures and energies in the FeMo cofactor cluster in nitrogenase, and the influence of the environment on redox properties, in comparison also with other simpler biological and synthetic iron-sulfur centers. Calculations and analysis of vibrational spectra and of hyperfine spectra compared with experiment are important indicators of stable intermediates over the catalytic cycle. All changes are in italics.